Polybrominated biphenyl mixture (Firemaster FF-1) was given orally to rats and mice at 0.1, 0.3, 1.0, 3.0 and 10.0 mg/kg body weight, 125 total doses over a 6-month period. After the 6-month exposure to PBB, the animals were held for an additional 2-year period for lifetime observation. Polybrominated biphenyl mixture (Firemaster FF-1) was carcinogenic for Fischer 344 rats and B6C3F1 mice of both sexes. The higher incidence of hepatic neoplasms included neoplastic nodules, hepatocellular carcnomas and cholanoig-carcinomas in rats and hepatocellular carinomas in mice. Other toxicities included porphyrogenic effects, hepatotoxicity, chronic progressive nephropathy and hyperplastic gastropathy in rat. The PBB mixture also affected the body weight gain in male and female rats and male mice although there was no significant difference in food consumption.